1. Field of the Invention
This invention relates to the use of 2-oxazoline cross-linking agents for effecting the ambient temperature curing of a reactive vinyl addition polymer. More particularly, the invention relates to the use of said cross-linking agents for preparing pressure-sensitive adhesive films from an acrylic interpolymer.
2. Description of the Prior Art
Pressure-sensitive adhesive resins are widely employed in a form of a film-like coating on a variety of superstrates in order to bond the latter to a normally non-adhering substrate. Another important use is in fabricating decorative laminations, such as decals. In general, pressure-sensitive resins for these applications must exhibit permanent tackiness and at the same time possess a requisite high degree of bonding capability; i.e., adhesiveness as well as cohesiveness. These properties can be built into an acrylic polymer to a large extent by the appropriate choice of constituent monomers and hence such polymeric compositions or interpolymers are preferred, especially for preparing premium quality products.
The cohesive strength of a pressure-sensitive adhesive resin represents a particularly critical property. While certain vinyl monomers inherently contribute to this property, the molecular weight of the polymer by and large is the governing factor in this regard. In practice, however, the molecular weight requirement poses problems, particularly in respect of the method conventionally observed for applying a coating of the adhesive to the superstrate. This is so because the current coating procedures require the use of an organic solution of the polymer having a manageable viscosity at a reasonably high solids content. Neither of the latter desiderata can be realized if the resin itself is of a molecular weight sufficiently high in order to provide the necessary cohesive strength.
The prior art has proposed several expedients for overcoming the need to apply resin solutions of impractically high viscosity. The expedient most prevalently observed at present consists of coating the superstrate with an organic solution of a reactive acrylic interpolymer of moderately high molecular weight and a cross-linking agent therefor. The applicable interpolymers are those having randomly distributed pendent carboxyl, hydroxyl, amino or acid anhyride groups as potential reactive sites. The currently preferred cross-linking agents are the chelates of a variety of metals, especially the chelates of titanium wherein the ligand moeity comprises ester groups. In a system of this type, the metal chelate and the interpolymer react in a progressive manner. Consequently, the film coating operation does not become seriously hampered at any time prior to the evaporation of the solvent.